The common assignee of the subject application is an industry leader in the design and manufacture of ultrasonic measurement devices. Such ultrasonic measurement devices measure the distance between an object and a transducer by sending a high frequency pulse to the object and capturing echoes reflected by the object. By measuring the delay between the transmission of the high frequency pulse and the reception of the echo(s), the distance between the object and the transducer is determined.
In many applications, the ultrasonic device will include several transducers. Multiple ultrasonic transducers allow multiple distances to be measured at the same time. The transducers are coupled to a single electronic control circuit through a series of relays. The relays function as a multiplexer. A transducer is selected, i.e. coupled to the electronic circuit, by activating the associated relay, while the other relays serve to isolate the unselected transducers, thereby preventing cross-talk or interference between transducers. Conventional relays are able to switch high currents and voltages as both lternating current (AC) and direct current (DC). Conventional electromechanical relays also feature a very high galvanic isolation in the order of 10.sup.12 Ohms. The high degree of isolation facilitates multiplexing multiple transducers without giving rise to significant cross-talk between transducers.
To improve the performance of the ultrasonic measurement devices, a temperature sensing circuit 204 is included. Temperature changes in air, gas, water, liquid, etc. will affect the speed of the ultrasonic sound waves, and the temperature sensing circuit allows the effects of temperatures to be taken into account. The temperature sensing circuit measures the temperature inside the transducers, and generates a current proportional to the absolute temperature (1 uA/.degree.K). Because the output from the sensing circuit comprises a current having a typical value between 233 uA for -40.degree. C. and 423 uA for +150.degree. C., it is critical that leakage currents from the other transducers be carefully controlled, otherwise the temperature readings will be inaccurate. For many applications, the ultrasonic measurement device will utilize 10 ultrasonic transducers. Consequently, in order to accurately read a temperature sensing circuit the leakage current contribution from all the transducers must not exceed 0.1 uA, otherwise, the leakage currents may begin to affect the reading from the temperature sensing circuit for the active transducer.
Conventional electromechanical relays utilize mechanical contacts for switching and isolation. While electromechanical relays are capable of providing the required level of isolation between the transducers, the operating life of the relay depends on the mechanical contacts. Repetitive switching of an electromechanical relay causes the contacts to wear. For certain applications of the ultrasonic measurement device, such as grain level monitoring in silos or liquid chemical measurement in a tank, the contacts are more susceptible to degradation due to airborne dust particulate or caustic chemical vapours. In such industrial applications, the chemical vapours will tend to oxidize or ionize the conductive coatings which are applied to the surfaces of the contacts. As the integrity of the contacts deteriorates the current is not able to flow through the relay. As a result, echo and temperature measurements cannot be read. It has been found that in an industrial application with the presence of ammonia vapour, conventional electromechanical relays need to be replaced as soon as 2 months for the ultrasonic measurement device to function properly. Another drawback of electromechanical relays is the switching noise which accompanies the actuation of the contacts.
Accordingly, there remains a need for a suitable replacement for conventional electromechanical relays which provides the isolation characteristics of electromechanical devices while at the same time is resistant to the effects of harsh environments.